1. Field of the Invention
This invention relates to a pulse modulator and more particularly to a fast-switching pulse modulator for applying a bias signal to a microwave apparatus having a TRAPATT or GaAs IMPATT-type diode.
2. Description of the Prior Art
The operation of a microwave amplifier or oscillator having a high efficiency avalanche diode such as a TRAPATT diode or a gallium-arsenide (GaAs) IMPATT diode requires a high-power pulse generator to provide the pulse power to the diode. Conventional generators, for example, commercially available under the tradename "Velonex", which provide a high pulsed-power output from a low-power input are typically bulky and costly and are primarily used for laboratory purposes.
In an effort to reduce the size of the pulse generators and make them cost-effective, application of pulse power to high efficiency avalanche diodes in the prior art is achieved by a modulator utilizing one or more transistors. A low-power modulator using a transistor which is capable of delivering large pulse power is described in an article entitled "Low-Cost Pulsing of Avalanche Diodes" by A. S. Clorfeine, R. D. Hughes and S. Weisbrod published in the RCA Review, Vol. 32, No. 3, September, 1971. A d.c. voltage is applied to the diode concurrent with the pulse.
Other prior art modulators have been developed which make use of the characteristics of the TRAPATT diode or GaAs IMPATT diode in a microwave apparatus in an attempt to provide a lower voltage pulse than that provided by conventional pulsers. The lower voltage requirement allows for smaller, simpler and thus less expensive pulsers. For example, according to the present understanding of the principles of operation, a TRAPATT diode requires a fast-rising threshold voltage (on the order of twice the breakdown voltage) to initiate the diode into the TRAPATT mode of operation when the diode is placed in a suitable circuit. (See, for example, W. J. Evans, "Circuits for High Efficiency Avalanche-Diode Oscillators", IEEE Trans. Microwave Theory Tech., Vol. MTT-17, pp. 1060-1067, Dec. 1969.) Typically, in the case of an oscillator circuit, the diode is reverse biased by a pulsed or D.C. voltage. The TRAPATT mode can be initiated by a fast-rising pulse or IMPATT oscillation. For an amplifier, the diode is usually reverse biased at a lower voltage than the threshold voltage. Application of an RF input signal of sufficient amplitude will combine with the bias signal to trigger the diode into the TRAPATT mode. Upon meeting the threshold conditions for the high efficiency mode of either the TRAPATT diode or GaAs IMPATT diode, the diode operation voltage (average d.c. voltage) drops sharply and a large current (average d.c. current) is drawn through the diode creating a negative resistance of the diode. The prior art modulators make use of the fact that below its breakdown voltage, a high efficiency avalanche diode is an open circuit and can be D.C. biased in that region. (See, A. Rosen, J. F. Reynolds, S. G. Liu, and G. E. Theriault, "Wideband Class-C Trapatt Amplifiers," RCA Review, Vol. 33, pp. 729-736, 1972.) Thus, a lower voltage from a pulser can be used to trigger the diode into the high efficiency mode of operation when the diode is pre-biased below the threshold voltage.
A disadvantage of these prior art modulators is in the rise time of the RF output pulse of the microwave apparatus. It is often desirable that the rise time of the output pulse be as rapid as possible, in particular, in systems where the period of the pulsed signal is very short. Generally, the switching time of the output pulse of the apparatus is limited by the rise time of the pulse of the current through the diode. Typically, the current pulse is switched from zero to its desired value as the diode is triggered into the high efficiency mode generating thereby an RF output pulse, such an increase in current resulting in a relatively slow rise time of the output pulse.